Acute severe asthma, formerly known as status asthmaticus, is defined as severe asthma unresponsive to repeated courses of beta-agonist therapy such as inhaled albuterol, levalbuterol, or subcutaneous epinephrine. It is a medical emergency that requires immediate recognition and treatment. Oral or parenteral corticosteroids should be administered to all patients with acute severe asthma as early as possible because clinical benefits may not occur for a minimum of 6-12 hours. Approximately 50% of episodes are attributable to upper respiratory infections, and other causes include medical nonadherence, nonsteroidal anti-inflammatory exposure in aspirin-allergic patients, allergen exposure (especially pets) in severely atopic individuals, irritant inhalation (smoke, paint, etc.), exercise, and insufficient use of inhaled or oral corticosteroids. The patient history should be focused on acute severe asthma including current use of oral or inhaled corticosteroids, number of hospitalizations, emergency room visits, intensive-care unit admissions and intubations, the frequency of albuterol use, the presence of nighttime symptoms, exercise intolerance, current medications or illicit drug use, exposure to allergens, and other significant medical conditions. Severe airflow obstruction may be predicted by accessory muscle use, pulsus paradoxus, refusal to recline below 30°, a pulse >120 beats/min, and decreased breath sounds. Physicians' subjective assessments of airway obstruction are often inaccurate. More objective measures of airway obstruction via peak flow (or forced expiratory volume in 1 second) and pulse oximetry before oxygen administration usually are helpful. Pulse oximetry values >90% are less commonly associated with problems although CO(2) retention and a low Pao(2) may be missed.